Hardening of 42CrMo4

You need to provide your data in an understandable fashion. if the material is plate, How can you describe it as 250 X 25 thk? Are we talking millimeters? A plate requires width thickness and length to adequately describe its dimensional attributes.

Is this a 25 mm thick 250 mm diameter circular cut out? or is this a 250 long by 25 dia shaft and the word plate a misnomer? Again, millimeters, right?

25mm is nominally equivalent to 1 inch.

42CRMo4 is nominally equivalent to US 4140/4142.

350Bhn is roughly equivalent to 37-38 Hrc; My read of the H-band using even a mild oil quench shows through hardening to core of a 2" diameter (~50 mm rd)at that hardness (J-12 for those of you following along at home in your ASTM A 304 book)

Since this will quench out in the high 50's to low 60's Hrc, you will achieve the target hardness by tempering back to your desired level. Presuming 25mm sectional thickness, I would austenitize in a furnace at 1550degrees F, temper to 800-900 degrees F to get the 350 BHN you seek. (According to my notes, at 800 degrees it'll probably hit 385 Bhn; 1000 degrees- ~320 Bhn).

Your mileage may vary.

If you use induction and water quench, that is more aggressive and you may need higher temper temperature. Problem with induction on this grade is getting adequate 'soak' time, you have to get the whole section to transform to austenite before you hit it with the water. that takes tiime. and penetration. and time at penetration for the core. I expect 15% parts crack if you use induction and water, water /polymer.

I like induction for surface hardening. I think it sucks for through hardening.

my opinion.

If this is not a 25mm thick maximum section, please do not use my comments!

milo